Dr. Copper, as the industrial metal is known on Wall Street, has fallen 6% this year, while the Dow industrials have zipped higher by 10%. This is not a good sign, since the last few years have seen copper and stocks generally move together: when people are feeling good about the global economy, both tend to benefit. If copper is weak, go the worrywarts, won’t stocks be close behind?

Krinsky used to be one of these worrywarts. But upon further inspection, he argues in a note this morning to his clients, copper’s real relationship is not to stocks — it’s to the dollar. The two are inversely correlated, according to history, for reasons that make sense: Because copper is denominated in dollars, a rise in the dollar makes copper more expensive for buyers in other currencies.

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